At first blush, you notice the two lenses are very similar – Bigron is only slightly longer and about the same weight. The filter diameter, and therefore front element, is also larger on Bigron: 95mm versus 86mm on the Sigma. Aside from a weather-sealing rubber o-ring, most of the differences are internal.

Both lenses are smaller than many of the super telephoto primes from Canon and Nikon, and very hand-holdable.

The following parts of this test will focus on a few things: a controlled environment to look at sharpness and vignetting and impressions and samples from three days of birding.